IB Biology Genetics 2015

Topic Six: Genetics
http://commons.wikimedia.org/wiki/File:Hall_of_Biodiversity,_American_Museum_of_Natural_History_(7356570500).jpg
Hall of Biodiversity

3.1 Genes
• Essential idea: Every living organism inherits
a blueprint for life from its parents.
http://pixabay.com/static/uploads/photo/2014/05/11/14/25/elephants-341981_640.jpg

Understandings
Statement Guidance
3.1 U.1
A gene is a heritable factor that consists of a length
of DNA and influences a specific characteristic
3.1 U.2
A gene occupies a specific position on a
chromosome.
3.1 U.3 The various specific forms of a gene are alleles.
3.1 U.4
Alleles differ from each other by one or only a few
bases.
3.1 U.5
New alleles are formed by mutation. [Deletions,
insertions and frame shift mutations do not need to
be included.]
3.1 U.6
The genome is the whole of the genetic information
of an organism.
3.1 U.7
The entire base sequence of human genes was
sequenced in the Human Genome Project.

Applications and Skills
Statement Guidance
3.1 A.1
The causes of sickle cell anemia, including a base substitution mutation, a change to
the base sequence of mRNA transcribed from it and a change to the sequence of a
polypeptide in hemoglobin. [Students should be able to recall one specific base
substitution that causes glutamic acid to be substituted by valine as the sixth amino
acid in the hemoglobin polypeptide.]
3.1 A.2
Comparison of the number of genes in humans with other species. [The number of
genes in a species should not be referred to as genome size as this term is used for
the total amount of DNA. At least one plant and one bacterium should be included
in the comparison and at least one species with more genes and one with fewer
genes than a human.]
3.1 S.1
Use of a database to determine differences in the base sequence of a gene in two
species. [The Genbank® database can be used to search for DNA base sequences.
The cytochrome C gene sequence is available for many different organisms and is of
particular interest because of its use in reclassifying organisms into three domains.]

3.1.U.1 A gene is a heritable factor that consists of a length of DNA and
influences a specific characteristic
Eukaryotic Chromosomes
• The Photograph shows a
human chromosome, which
are made of DNA and
protein. The picture show
two chromatids (one copy of
a newly replicated
chromosome) joined at the
centromere. Each Eukaryotic
chromosomes is composed
of genes which control
specific characteristics
(Alleles are different forms
of a specific gene)
Gene Locus: specific position of a
gene on a chromosome

Chromosome
Histones
DNA double helix
3.1 U.1 A gene is a heritable factor that consists of a length of DNA and
influences a specific characteristic
Chromosome is a length of DNA with associated proteins (histones).

Homologous Pair of Chromosomes:
One Comes From Each Parent
•Gene loci is the position of the gene on a chromosome. Alleles have the same
gene loci.

A gene can exist in many different forms, called alleles. For example, let’s say that
there is one gene which determines the color of your hair. That one gene may
have many forms, or alleles: black hair, brown hair, auburn hair, red hair, blond
hair, etc. You inherit one allele for each gene from your mother and one from your
father.
Each of the two alleles you inherit for a gene each may be strong ("dominant") or
weak ("recessive"). When an allele is dominant, it means that the physical
characteristic ("trait") it codes for usually is expressed, or shown, in the living
organism. You need only one dominant allele to express a dominant trait. You
need two recessive alleles to show a recessive form of a trait. See the heredity
diagram for tongue rolling to see how dominant and recessive alleles work.

3.1.A2 Comparison of the number of genes in humans with other species. [The
number of genes in a species should not be referred to as genome size as this term is
used for the total amount of DNA. At least one plant and one bacterium should be
included in the comparison and at least one species with more genes and one with
fewer genes than a human.]

3.1 U.2 A gene occupies a specific position on a chromosome.
http://www.daviddarlin
g.info/images/gene.jpg
•Gene is a heritable factor that controls a specific
characteristic. There are over 20,000 genes that make up a
human. One gene codes for one polypeptide.

Allele is one specific
form of a gene, differing
from other alleles by
one or a few bases only
and occupying the same
gene locus as other
alleles of the gene.
3.1 U.3 The various specific forms of a gene are alleles.

3.1 U.6 The genome is the whole of the genetic information of an
organism.
Allows for:
• Physical mapping of chromosomes
• Used to screen for genetic diseases
• Lead to a better understanding of genetic diseases.
• May lead to the development of better drugs to fight diseases.
• Maybe be used for comparison of genomes with other species.

organism.
Genome Size and Number of Genes

Chromosome 16
organism.
http://physrev.physiology.org/content/physrev/91/1/151/F11.large.jpg

organism.
Advantages from the project
Advantage 1
• Screening can also be done for
adults, to see if they may be
carriers of potential genetic
conditions. Certain Jewish and
Canadian populations regularly
obtain voluntary screening for Tay-
Sachs disease, a known child-killer.
This information has been used to
help make decisions about future
marriage partners.
Advantage 2
Perhaps the greatest benefit will come
from what is called gene-based
therapy. Understanding the
molecular workings of genes and
the proteins they encode will lead
to more precise drug treatments.
The more precise the drug
treatment, the fewer and milder
will be the side effects.

3.1 U.7 The entire base sequence of human genes was sequenced in the
Human Genome Project.
• The Human Genome Project (HGP) was an international scientific research project with the
goal of determining the sequence of chemical base pairs which make up human DNA, and of
identifying and mapping all of the genes of the human genome from both a physical and
functional standpoint. It remains the world's largest collaborative biological project
• Key findings of the draft (2001) and complete (2004) genome sequences include:
 There are approximately 20,500 genes in human beings, the same range as in mice.
 Significantly more segmental duplications (nearly identical, repeated sections of DNA)
than had been previously suspected.
 At the time of publishing fewer than 7% of protein families appeared to be vertebrate
specific.

3.1 U.7 The entire base sequence of human genes was sequenced in the
Human Genome Project.
http://www.bioscience.heacademy.ac.uk/imagebank/search/Fullimage.aspx?IDvalues=6513
Key advances in technology:
• Biotechnology techniques such as PCR are used to prepare samples: the DNA needs
to be copied to prepare a sufficiently large pure samples to sequence
• Computers automate the sequencing process
• Fluorescent labeling techniques enable all four nucleotides to be analyzed together
• Lasers are used to fluoresce the dye markers
• Digital camera technology reads the dye markers
• Computers are used to assemble the base sequence

3.1 U.4 Alleles differ from each other by one or only a few bases.
This may cause:
•Change in the base sequence of the gene
•Change in the mRNA made in transcription
•Change in the amino acids sequence / primary structure of the protein
•Change in the secondary/ tertiary or quaternary structure of the
protein
•Change the shape of the protein
•Change the function of the protein.
http://evolution.berkeley.edu/evolibrary/images/interviews/pax_mouse_fly.gif

Examples of the mutations to the gene
Deletion
Duplication
Inversion
Translocation
3.1 U.5 New alleles are formed by mutation. [Deletions, insertions and
frameshift mutations do not need to be included.]
http://www.goldiesroom.org/Multimedia/Bio_Images/19%20Applie
d%20Genetics/05%20Chromosome%20Mutations.jpg

3.1 U.5 New alleles are formed by mutation.

3.1 A.1 The causes of sickle cell anemia, including a base substitution mutation, a change to the
base sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in
hemoglobin. [Students should be able to recall one specific base substitution that causes
glutamic acid to be substituted by valine as the sixth amino acid in the hemoglobin
polypeptide.]

3.1 A.1 The causes of sickle cell anemia
The Disease:
•Sickle cell anemia is an inherited disorder
that affects hemoglobin, a protein that
enables red blood cells to carry oxygen
to all parts of the body.
•The disorder caused by a base
substiution mutation, produces
abnormal hemoglobin, which along
with the abnormalities of the cells
membrane cause damage from be
trapped and freed in the capillaries.
This shortens the blood cells life to as
little as 4 days.
Disease Symptoms:
•chronic anemia,
•acute chest syndrome,
•stroke,
•spleen and renal dysfunction,
•pain crises and susceptibility to bacterial
infections, particularly in children.
•Sickle cell disease is also associated with
significant mortality.

•The HBA gene carries the
instructions for the manufacture
of a protein that is a component
of hemoglobin.
•Hemoglobin is the protein
containing a sequence GAG
which codes for the amino acid
glutamic acid.
•In a base substitution mutation of
Hemoglobin proteins in Sickle
Cell Anemia. GAG has been
changed into GTG which codes
for the amino acid valine. It is
given the symbol HBs for the
gene.
http://upload.wikimedia.org/wikipedia/commons/a/ac/Sickle_cell_01.jpg

Alleles, genotypes and Phenotypes of Sickle Cell:
HbA Normal allele producing normal protein using the amino acid
glutamic acid
Hbs Sickle allele which is the abnormal mutation of the normal allele. Is
produced using the replacement amino acid valine.
http://www.zo.utexas.edu/faculty/sjasper/images/17.23.gif

3.1.A1 The causes of sickle cell anemia, including a base substitution mutation, a change to the base
sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in hemoglobin.

World Sickle allele Distribution:
Compare the frequency distribution of sickle allele with the
distribution of malaria.
•Notice that the high frequency of sickle coincides with
malaria regions

•In these regions sickle cell trait ( HbAHbs ) are resistant to the
malaria parasite. This is because the Hbs allele makes it difficult
for the parasite to live inside the red cells. Sickle cell trait
(carriers) therefore survive malaria infection.
•HbAHbA normal hemoglobin people are susceptible to malaria
infection and do not survive well.
•HbsHbs have sickle disease and do not survive well.
•The sickle allele survives well in malaria regions accounting for its
high frequency in these regions.
•With migration the allele has spread around the world.
Compare the frequency distribution of sickle allele with the
distribution of malaria.

3.1.S1 Use of a database to determine differences in the base sequence
of a gene in two species.
One use of aligning base sequences is to determine
the differences between species: this can be used to
help determine evolutionary relationships.
http://www.ncbi.nlm
.nih.gov/genbank
GenBank
http://bitesizebio.s3.amazonaws.com/wp-content/uploads/2012/10/header-image-copy18.jpg
Your task is to analyze the differences between three or more species
(the skill asks for two species, but the online Clustal tool works better
with a minimum of three).
For each chosen species retrieve the base sequence:
• Go to GenBank website http://www.ncbi.nlm.nih.gov/genbank
• Select ‘Gene’ from the search bar
• Enter the name of a gene (e.g. AMY1A for salivary amylase 1A or COX1 for
cytochrome oxidase 1) AND the organism (use the binomial) and press ‘Search’
n.b. if you are comparing species the gene chosen needs to be the same for each
species
• Select the ‘Name/Gene ID’ to get a detailed view
• Scroll down to the ‘Genomic regions, transcripts, and products’ section and click on
‘FASTA’
• Copy the entire sequence from ‘>’ onwards
• Save the sequence – you will need to align with the other species next

3.1.S1 Use of a database to determine differences in the base sequence
of a gene in two species.
http://www.ebi.ac.uk/Tools/msa/clustalo/
Analysis:
• ‘Alignments’ allows you to visually check the results – this is easier
if the chosen gene has a short base sequence
• Under ‘Results Summary’ use the ‘Percent Identity Matrix’ to
quantify the overall similarity (0 = no similarity, 100 = identical)
• Under ‘Phylogenic Tree’ chose the ‘Real’ option for the Phylogram
to get a visual representation of how similar the species are (based
on the chosen gene).
http://bitesizebio.s3.amazonaws.com/wp-content/uploads/2012/10/header-image-copy18.jpg
To align the sequences:
• Go to the Clustal Omega website
http://www.ebi.ac.uk/Tools/msa/clustalo/
• In STEP 1 Select ‘DNA’ under ‘a set of’
• Paste the chosen sequences into the box
(each sequence must start on a new line)
• Press ‘Submit’ (and wait – depending on
the size of the sequences you may have to
wait for a couple of minutes)

3.2 Chromosomes
Essential
Question:
Chromosomes
carry genes in
a linear
sequence that
is shared by
members of a
species.
http://41.media.tumblr.com/4c08ea6
94bcf30c5a9d8423187fba2de/tumblr
_mx5zd49kWN1qjofuoo1_1280.jpg
Stages of Mitosis by Walter Flemming January 1882

Understandings
Statement Guidance
3.2 U.1 Prokaryotes have one chromosome consisting of a circular DNA molecule.
3.2 U.2 Some prokaryotes also have plasmids but eukaryotes do not.
3.2.U.3 Eukaryote chromosomes are linear DNA molecules associated with histone proteins.
3.2 U.4 In a eukaryote species there are different chromosomes that carry different genes.
3.2.U.5
Homologous chromosomes carry the same sequence of genes but not necessarily the same
alleles of those genes.
3.2 U.6 Diploid nuclei have pairs of homologous chromosomes.
3.2 U.7
Haploid nuclei have one chromosome of each pair. [The two DNA molecules formed by DNA
replication prior to cell division are considered to be sister chromatids until the splitting of the
centromere at the start of anaphase. After this, they are individual chromosomes.]
3.2 U.8 The number of chromosomes is a characteristic feature of members of a species.
3.2 U.9
A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.
[The terms karyotype and karyogram have different meanings. Karyotype is a property of a cell—
the number and type of chromosomes present in the nucleus, not a photograph or diagram of
them.]
3.2 U.10
Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine
sex.

Statement Guidance
3.2 A.1
Cairns’ technique for measuring the length of DNA molecules by
autoradiography.
3.2 A.2
Comparison of genome size in T2 phage, Escherichia coli, Drosophila
melanogaster, Homo sapiens and Paris japonica. [Genome size is the total
length of DNA in an organism. The examples of genome and chromosome
number have been selected to allow points of interest to be raised
3.2 A.3
Comparison of diploid chromosome numbers of Homo sapiens, Pan
troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.
3.2 A.4 Use of karyograms to deduce sex and diagnose Down syndrome in humans.
3.2 S.1
Use of databases to identify the locus of a human gene and its polypeptide
product.

3.2 U.1 Prokaryotes have one chromosome consisting of a circular DNA
molecule.
Prokaryotes have two types of DNA: a single chromosome
and plasmids (not associated with any of the cells life
processes
• Prokaryotic DNA is circular and is not associated with any
histone proteins (“naked DNA)
• There is one copy of each gene except when the cell and
its DNA are replicating
• The single chromosome is coiled up and concentrated in
the nucleoid region.
Bacterial
DNA
Plasmids

3.2 U.1 Prokaryotes have one chromosome consisting of a circular DNA
molecule.
https://classconnection.s3.amazonaws.com/730/flashcards/127
6730/jpg/cell_types1330806303645.jpg

3.2 U.2 Some prokaryotes also have plasmids but eukaryotes do not.
http://upload.wikimedia.org/wikipedia/en/6/6c/PBR322_pl
asmid_showing_restriction_sites_and_resistance_genes.jpg
Features of Plasmids:
• Naked DNA - not associated with histone
proteins
• Small circular rings of DNA
• Can be passed between prokaryotes
• Plasmids are small separate (usually
circular) DNA molecules located in some
prokaryotic cells
• Plasmids are also naked (not associated
with proteins) and are not needed for
daily life processes in the cell.
• The genes in plasmids are often
associated with antibiotic resistant and
can be transferred from one bacterial cell
to another.
• Plasmids are readily used by scientists to
artificially transfer genes from one species
to another (ie. Gene for human insulin)

Nucleosome Structure: consist of 8 histones. They help create super coiling of
chromatin, which creates a chromosome during cell replication
3.2 U.3 Eukaryote chromosomes are linear DNA molecules associated
with histone proteins.
http://micro.magnet.fsu.edu/cells/nucleus/images/
chromatinstructurefigure1.jpg

• Eukaryotic chromosomes are linear
and are made up of DNA and
histone proteins, important in gene
expression and replication.
• Histones are globular shaped
protein in which the DNA is
wrapped around.
• DNA wrapped around 8 histone
proteins is called a nucleosome.
• The DNA wraps twice around the
histone protein core.
• Another histone protein is attached
to the outside of the DNA strand.
This helps maintain the colloidal
structure of the nucleosome.
• DNA, because of its negative charge
is attracted to the positive charge
on the amino acids of the histone
proteins.
3.2 U.3 Eukaryote chromosomes are linear DNA molecules associated
with histone proteins.
http://upload.wikimedia.org/wikipedia/commons/
4/45/Nucleosome_organization.png

3.2 U.4 In a eukaryote species there are different chromosomes that carry
different genes.
https://s-media-cache-
ak0.pinimg.com/originals/4d/d0/7
a/4dd07a61c384abefef3c521bcab
6bad8.jpg
http://dxline.info/img/new_ail/chro
mosome-4.jpg
• Chromosomes are
linear, varying in length
and in position of the
centromere that holds
the sister chromatids
together.
• In humans there are 23
types of chromosomes.
Each chromosome
carries a specific
sequence of genes
along the linear DNA
molecule. The position
where the gene is
located is called the
locus.
• Alleles are different
forms of a gene on the
chromosome.
Chromosome 1 Chromosome 4
2,000 Genes 1,000 Genes

https://public.ornl.gov/site/gallery/originals/
Eukaryotes possess multiple chromosomes. All
individuals of a species possess the same
chromosomes, with the same gene loci. For
example all humans have twenty three pairs.
Chromosomes can vary by:
• Length – the number of base pairs in the DNA molecule
• Position of the centromere
• Genes occur at a specific locus (location), i.e. it is always
found at the same position on the same chromosome
(the locus and genes possessed vary between species)
3.2 U.4 In a eukaryote species there are different chromosomes that
carry different genes.

3.2 U.5 Homologous chromosomes carry the same sequence of genes
but not necessarily the same alleles of those genes.
https://molecularhelix.files.wordpress.com/2011/07/homolg.gif
• Homologous chromosomes
are chromosomes within each
cell that carry the same genes
• One chromosome came from
an individual’s mother and
one from the father
• They have the same shape
and size
• These chromosomes pair up
during meiosis
• Even though these
chromosomes carry the same
genes, they could have
different alleles (different
versions of the same gene)

3.2 U.6 Diploid nuclei have pairs of homologous chromosomes.
• Diploid nuclei have two copies of
each type of chromosome. One
chromosome comes from the
mother and one from the father.
• Haploid gametes (sperm and egg)
fuse during sexual reproduction
which produces zygote with a
diploid nucleus
• This cell will then divide by
mitosis to produce numerous
cells, all with a diploid nucleus
• Each nucleus has two copies of
each gene, accept the sex
chromosomes

3.2 U.7 Haploid nuclei have one chromosome of each pair. [The two
DNA molecules formed by DNA replication prior to cell division are
considered to be sister chromatids until the splitting of the centromere
at the start of anaphase. After this, they are individual chromosomes.]
• Haploid nuclei have one copy of each chromosome or one full set of the
chromosomes (Sex cells) in that particular species eg. Human 23 chromosomes
• These are called gametes, which are sperm and egg
• Human sperm and eggs each contain 23 chromosomes (when the fuse together the
become diploid with 46 chromosomes, a somatic body cell)
https://classconnection.s3.amazonaws.com/116215/flashcards/771420/png/meiosis-1.png

3.2 U.7 Haploid nuclei have one chromosome of each pair. [The two
DNA molecules formed by DNA replication prior to cell division are
considered to be sister chromatids until the splitting of the centromere
at the start of anaphase. After this, they are individual chromosomes.]
• Chromosome to the right
splits at the centromere.
• This occurs during Anaphase
• The chromatids move to
opposite poles of the cell to
become chromosomes in a
newly created cell.
Chromosome
With two
Chromatids
Become Two
Chromosome

3.2 U.8 The number of chromosomes is a characteristic feature of
members of a species.
• The chromosome number is a characteristic
feature of that species 2 (n) in a diploid cell or
n in a haploid cell.
• A chromosome number does not indicate how
complicated an organism might be
• Organisms with different numbers of
chromosomes which is why the letter n is
used because it is a variable and they might
not be able to interbred
• Chromosome number tends to remain
unchanged over millions of years of
evolution; however, sometimes through
evolution chromosomes can fuse together or
split to change the number of chromosomes
an organism contains
• During human evolution, two ancestral ape
chromosomes fused to produce human
chromosome 2

3.2 U.9 A karyogram shows the chromosomes of an organism in
homologous pairs of decreasing length.
https://commons.wikimedia.org/wiki/File:NHGRI_human_male_karyotype.png

3.2 U.9 A karyogram shows the chromosomes of an organism in homologous pairs of
decreasing length.
http://learn.genetics.utah.edu/content/chromosomes/karyotype/
Karyotype is a property of the cell described by the number and type of chromosomes
present in the nucleus (of a eukaryote cell).
Karyogram is a diagram or photograph of the chromosomes present in a nucleus (of a
eukaryote cell) arranged in homologous pairs of decreasing length.
a Karyogram is a diagram
that shows, or can be used to
determine, the karyotype.

3.2 U.10 Sex is determined by sex chromosomes and autosomes are
chromosomes that do not determine sex.
• The X and Y chromosome determine the sex
of an individual
• The X chromosome contains over 2000 genes
in comparison to the Y chromosome has less
then 100 genes
• If an individual has two homologous X
chromosomes they will be a female and if
they have an two non homologous X and a Y
chromosome they will be a male
• All other chromosomes are called autosomes
and do not affect the sex of an individual
• The X chromosome has many genes located
on it essential to human development, while
the Y chromosome has a small number of
genes (some of these are shared with the X
chromosome). The rest of the genes on the Y
chromosome are only necessary for male
development
http://images.zeit.de/wissen/gesundheit/2014
-01/y-chromosom/y-chromosom-540x304.jpg

3.2 U.10 Sex is determined by sex chromosomes and autosomes are chromosomes that do not
determine sex.

3.2 A.1 Cairns’ technique for measuring the length of DNA molecules by
autoradiography.
• John Cairns first supplied the E. Coli bacteria
cells with suitable radioactive material
(replaces normal hydrogen in thymidine).
• Used to selectively label only DNA and will not
label RNA.
• Intact E. Coli bacterial chromosomes are placed
on slides. These slides are then covered by
photographic emulsion and stored in dark.
• During this storage the particles are emitted
the radioactive material, which exposed the
film
• The photographs show the regions of labelled
DNA.
Findings
• The images showed that E. coli possesses a
single circular chromosome which is 1,100 μm
long (E. coli cells have a length of only 2 μm)
• Cairns images also provided evidence to support
the theory of semi-conservative replication

3.2 A.2 Comparison of genome size in T2 phage, Escherichia coli, Drosophila
melanogaster, Homo sapiens and Paris japonica. [Genome size is the total length of DNA in an
organism. The examples of genome and chromosome number have been selected to allow
points of interest to be raised
Name Genome Length
(million base pairs)
Number of Genes
T2 phage (Virus) 0.18 300
Escherichia coli
(Bacteria)
5 4,377
Drosophila
melanogaster
(Fruit Fly)
140 17,000
Paris japonica
(Woodland Plant)
150,000 Unknown
Homo sapiens
(Human)
3,000 19-23,000

3.2 A.2 Comparison of genome size in T2 phage, Escherichia coli, Drosophila
melanogaster, Homo sapiens and Paris japonica. [Genome size is the total length of DNA in an
organism. The examples of genome and chromosome number have been selected to allow
points of interest to be raised
Paris japonica
Largest Known Genome

3.2 A.3 Comparison of diploid chromosome numbers of Homo
sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris
equorum.
Homo sapiens (46)

3.2 A.3 Comparison of diploid chromosome numbers of Homo sapiens, Pan
troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.
Pan troglodytes (48)
http://static1.squarespace.com/static/51b89f0ce4b0000660671282/t/540785e0e4b00aec95acf4cb/1409779168825/floandfigan.jpg

equorum.
Canis familiaris 78
https://c1.staticflickr.com/5/4007/5076413065_6a42962771_b.jpg

equorum.
Species Name Number of
Chromosomes
Homo sapiens 46
Pan troglodytes 48
Canis familiaris 78
Oryza sativa 24
Parascaris
equorum
2
Rice: Oryza sativa
http://upload.wikimedia.org/wikipedia
/commons/6/65/Oryza_sativa_Kyoto_J
PN_001.JPG
Round Worm:
Parascaris equorum
http://www.vetnext.com/fotos/quizc2a4.jpg

Normal Male
3.2 A.4 Use of karyograms to deduce sex and diagnose Down syndrome
in humans.
http://commons.wikimedia.org/wiki/File:NHGRI_human_male_karyotype.png

Normal Female
in humans.

Male with Down Syndrome: Trisomy 21; mental deficiencies ,
as well as physical abnormalities
in humans.
http://upload.wikimedia.org/wikipedia/commons/6/6f/Trisomie_21_Genom-Schema.gif

3.3 Meiosis
Essential Question: Alleles segregate during meiosis allowing
new combinations to be formed by the fusion of gametes.
http://i.huffpost.com/gen/1123387/original.jpg
Grandmother Granddaughter

Understandings
Statement Guidance
3.3.U.1 One diploid nucleus divides by meiosis to produce four haploid nuclei.
3.3 U.2
The halving of the chromosome number allows a sexual life cycle with
fusion of gametes.
3.3 U.3
DNA is replicated before meiosis so that all chromosomes consist of two
sister chromatids.
3.3.U.4
The early stages of meiosis involve pairing of homologous chromosomes
and crossing over followed by condensation. [The process of chiasmata
formation need not be explained.]
3.3 U.5
Orientation of pairs of homologous chromosomes prior to separation is
random.
3.3 U.6
Separation of pairs of homologous chromosomes in the first division of
meiosis halves the chromosome number.
3.3 U.7 Crossing over and random orientation promotes genetic variation.
3.3 U.8 Fusion of gametes from different parents promotes genetic variation.

Statement Guidance
3.3 A.1
Non-disjunction can cause Down syndrome and other chromosome
abnormalities.
3.3 A.2 Studies showing age of parents influences chances of non-disjunction.
3.3 A.3
Description of methods used to obtain cells for karyotype analysis e.g.
chorionic villus sampling and amniocentesis and the associated risks.
3.3 S.1
Drawing diagrams to show the stages of meiosis resulting in the formation
of four haploid cells. [Drawings of the stages of meiosis do not need to
include chiasmata. Preparation of microscope slides showing meiosis is
challenging and permanent slides should be available in case no cells in
meiosis are visible in temporary mounts.]

10.1 Meiosis
Essential idea: Meiosis leads to independent assortment of
chromosomes and unique composition of alleles in daughter cells.
http://upload.wikimedia.org/wikipedia/c
ommons/4/46/Dividing_Cell_Fluorescenc
e.jpg

Understandings
Statement Guidance
10.1 U.1 Chromosomes replicate in interphase before meiosis.
10.1 U.2 Crossing over is the exchange of DNA material between non-sister
homologous chromatids.
10.1 U.3 Crossing over produces new combinations of alleles on the chromosomes
of the haploid cells.
10.1 U.4 Chiasmata formation between non-sister chromatids can result in an
exchange of alleles.
10.1 U.5 Homologous chromosomes separate in meiosis I.
10.1 U.6 Sister chromatids separate in meiosis II.
10.1 U.7 Independent assortment of genes is due to the random orientation of pairs
of homologous chromosomes in meiosis I.

Statement Guidance
10.1 S Drawing diagrams to show chiasmata formed by crossing over.

Review: 1.6.U.1 Mitosis is division of the nucleus into two genetically identical daughter nuclei.
http://highered.mheducation.com/sites/0072495
855/student_view0/chapter2/animation__mitosis
_and_cytokinesis.html
Use the animated tutorials to learn about mitosis
http://www.johnkyrk.com/mitosis.html
http://www.sumanasinc.com/webcontent/animations/content
/mitosis.html
http://outreach.mcb.harvard.edu/animations/cellcycle.
swf

3.3 U.1 One diploid nucleus divides by meiosis to produce four haploid
nuclei.
http://cis.payap.ac.th/wp-content/uploads/2011/11/142.png
• Cells divide twice
• Result: 4 daughter cells, each
with half as many chromosomes
as parent cell

• What if a complex multicellular organism (like
us) wants to reproduce?
– joining of egg + sperm
• Do we make egg & sperm by mitosis?
46 46+ 92
egg sperm zygote
What if we did, then….
Doesn’t work!
No!
3.3 U.2 The halving of the chromosome number allows a sexual life cycle
with fusion of gametes.

3.3 U.1 One diploid nucleus divides by meiosis to produce four haploid nuclei.
Edited from: https://commons.wikimedia.org/wiki/File:Diagram_of_meiosis.svg
Meiosis is a reduction division of the nucleus to form haploid gametes
One diploid (2N) body cells contain a homologous
pair of each chromosome (except for sex cells)
Four haploid (N) gametes contain
one of each chromosome
Second division of the nucleus
First
division
of the
nucleus
Chromosomes are replicated to
form sister chromatids

3.3 U.1 One diploid nucleus divides by meiosis to produce four haploid nuclei.
http://www.sumanasinc.com/webcontent/animations/c
ontent/meiosis.html
http://highered.mheducation.com/sites/dl
/free/0072437316/120074/bio19.swf
http://www.stolaf.edu/people/giannini/fla
shanimat/celldivision/meiosis.swf
http://www.biostudio.com/d_%20Meiosis.htm
The animations
are a great way
to visualize the
process –
watch and take
notes.
Meiosis is a reduction division of the nucleus to form haploid gametes

3.3 U.2 The halving of the chromosome number allows a sexual life cycle
with fusion of gametes.
http://lh6.ggpht.com/n2T0xB_8BXE/UY4bhlV52uI/AAAAAAAABxg/7ZjWF
QMBfx4/s1600-h/Humanlifecycle9.jpg
Sexual life cycle
• Meiosis: reduces #
of chromosomes
creating gametes (n)
• Fertilization:
combine gametes
(sperm + egg)
creating a fertilized
egg or zygote (2n)

Duplication of Chromosomes During S stage of Interphase
Single chromosome
(called chromatin
during Interphase)
Two identical sister
chromatids joined by
a centromere
DNA replication during
S stage of Interphase
3.3 U.3 DNA is replicated before meiosis so that all chromosomes
consist of two sister chromatids.
.

10.1 U.1 Chromosomes replicate in interphase before meiosis.
• During the S phase of
the cell cycle, so that
each chromosome has a
copy of itself and
consists of two sister
chromatids.
• During meiosis I,
chromosomes condense
and synapse to form
bivalents (homologous
chromosomes are
aligned next to each
other).
http://www.columbia.edu/cu/biology/co
urses/c200507/images/meiosis_19.gif

Crossover
• Red and Blue chromosomes are a homologous pair.
• They have replicated during the interphase and each copy is
held together by the centromere (black dot)
3.3 U.4 The early stages of meiosis involve pairing of homologous
chromosomes and crossing over followed by condensation. [The process
of chiasmata formation need not be explained.]

•The chromosomes overlap and exchange lengths of DNA.
•The chiasmata are the points at which the red and blue chromosomes
overlap.
•At a molecular level the exchange has to be exact avoiding loss of
bases. Such an error would have profound effect on the genes.

• At anaphase I the homologous pairs are separated.
• Notice that sections of DNA have been exchanged.
• These will mean that the alleles on these section have also been
exchanged
• Crossing over, Independent Assortment, combined with Fertilization:
A couple can produce over 64 trillion (8.3 million x 8.3 million)
different combinations in a zygotes during fertilization

3.3 U.5 Orientation of pairs of homologous chromosomes prior to
separation is random.
http://cnx.org/resources/9d7cd91abd65e9c7fa857cf6c6d133ec/Figure_08_03_06.jpg
Independent assortment of chromosomes
• meiosis introduces genetic variation in several ways
• gametes of offspring do not have same combination
of genes as gametes from parents

10.1 U.2 Crossing over is the exchange of DNA material between non-
sister homologous chromatids.

10.1 U.3 Crossing over produces new combinations of alleles on the
chromosomes of the haploid cells.
http://www.indiana.edu/~oso/lessons/Genetics/
figs/sheep/2_fXe_sp2.jpg

10.1 U.4 Chiasmata formation between non-sister chromatids can result
in an exchange of alleles.
• Chiasmata are points where two
homologous non-sister chromatids exchange
genetic material during crossing over in meiosis.
• Chromosomes intertwine and break at the exact
same positions in non-sister chromatids.
• The two chromosomes are now attached at
the same corresponding position on the non-
sister chromatid.
• Once attached the non-attached portions of the
chromatids actually repel each other.
• Chiasmata refer to the actual break of the
phosphodiester bond during crossing over.
• The chiasmata are separated during anaphase 1
which can result in an exchange of
alleles between the non-sister chromatids from
the maternal and paternal chromosomes.

10.1 U.5 Homologous chromosomes separate in meiosis I.
• During meiosis I, unlike mitosis
homologous chromosomes separate
to opposite poles; however, their
sister chromatids remain attached
to each other
• Homologous chromosomes can
exchange material in a process
called crossing over
• Meiosis I is considered reduction
division because the chromosome
number is reduced by half (2n -> n
in humans)
http://i.ytimg.com/vi/RfRtVDkABPI/hqdefault.jpg

10.1 U.6 Sister chromatids separate in meiosis II.
• During meiosis II sister
chromatids separate (some are
non-identical sister chromatids
due to crossing over
• This type of separation is very
similar to mitosis as the
chromatids are separated from
each other

10.1 U.7 Independent assortment of genes is due to the random
orientation of pairs of homologous chromosomes in meiosis I.
• Independent assortment is an essential
component in explaining how
chromosomes align themselves during
meiosis.
• It also explains how unlinked genes are
passed on from generation to generation.
• When homologues line up along the
equatorial plate in metaphase I, the
orientation of each pair of is random;
meaning the maternal or paternal
homologue can orient towards either pole.
• The orientation of how one set of
homologues line up has no effect on how
any of the other homologues line up.
• Two young women to the right are non
identical twin sisters, who because of
independent assortment have different
hair and skin coloring
http://hellogiggles.com/bi-racial-twins/

10.1 U.7 Independent assortment of genes is due to the random
orientation of pairs of homologous chromosomes in meiosis I.

10.1 S.1 Drawing diagrams to show chiasmata formed by crossing over.
http://www.thestudentroom.co.uk/showthread.php?t=1561992
• The centromere is a section of
DNA on the chromosome where
the chromatids are closest, it's
generally tightly packed, the DNA
typically doesn't have a defined
sequence and is often satellite
DNA, non-coding. It's the point at
which the mitotic spindle attaches
itself.
• Chiasmata are places where the
chromosomes cross over, and
sometimes exchanges of DNA take
place. This is usually looser
packed, often functional DNA

3.3 U.6 Separation of pairs of homologous chromosomes in the first
division of meiosis halves the chromosome number.
• Each individual has a pair
chromosomes both
chromosomes of a pair carry
“matching” genes control same
inherited characters
homologous = same
information
• To create sex cells the number
of chromosomes must be
reduce for Humans from 46
chromosomes  to 23.
Reduced by half.
http://41.media.tumblr.com/4c08ea694bcf30c5a9d8423
187fba2de/tumblr_mx5zd49kWN1qjofuoo1_1280.jpg

3.3 S.1 Drawing diagrams to show the stages of meiosis resulting in the
formation of four haploid cells.
http://upload.wikimedia.org/wikipedia/commons/5/54/Meiosis_diagram.jpg

Meiosis 1
• interphase
• prophase 1
• metaphase 1
• anaphase 1
• telophase 1
Meiosis 2
• prophase 2
• metaphase 2
• anaphase 2
• telophase 2
2nd division of meiosis
separates sister
chromatids
(1n  1n)
* just like mitosis *
1st division of
meiosis separates
homologous pairs
(2n  1n)
“reduction division”

Meiosis: Generates haploid gametes
• Reduces the number of chromosomes by half.
• Also produces genetic variability, each gamete is different,
ensuring that two offspring from the same parents are
never identical.
• Two divisions: Meiosis I and meiosis II. Chromosomes are
duplicated in interphase prior to Meiosis I.

Meiosis I: Separation of Homologous Chromosomes
1. Prophase I:
• Chromatin condenses into chromosomes.
• Nuclear membrane disappear.
• Centrosomes move to opposite poles of cell and
microtubules attach to chromatids.
• Synapsis: Homologous chromosomes pair up and
form a tetrad of 4 sister chromatids.

Prophase I: Crossing Over Between Homologous Chromosomes
Crossing over combined with Fertilization: A couple can produce over 64
trillion (8.3 million x 8.3 million) different combinations in a zygotes during
fertilization.

Meiosis I:
2. Metaphase I:
3. Anaphase I:
4. Telophase I and Cytokinesis:

Random Assortment of Homologous Chromosomes
During Meiosis I Generates Many Possible Gametes

Meiosis II: Separation of Sister Chromatids
During interphase that follows meiosis I, no DNA replication
occurs.
Interphase may be very brief or absent.
Meiosis II is very similar to mitosis.
1. Prophase II:
• Very brief, chromosomes reform.
• No crossing over or synapsis.
• Spindle forms and starts to move chromosomes towards
center of the cell.

2. Metaphase II:
• Very brief, individual chromosomes line up in the middle
of the cell.
3. Anaphase II:
• Chromatids separate and move towards opposite ends of
the cell.

4. Telophase II:
• Nuclei form at opposite ends of the
cell.
• Cytokinesis occurs.
Product of meiosis:
Four (4) haploid gametes, each
genetically different from the other.

Outline the differences between the behaviour of
chromosomes in Mitosis and Meiosis
5 marks
Mitosis Meiosis
One division Two divisions
Diploid cells produced Haploid gametes produced
No crossing-over in prophase Crossing-over in prophase I
No chiasmata formation Chiasmata form
Homologous pairs do not associate and
line up at the equator in metaphase
Homologous pairs associate as bivalents
and lined up at the equator in metaphase I
Sister chromatids separate in anaphase
Homologous pairs separate in anaphase I
Sister chromatids separate in anaphase II

• Mendel’s Law of Independent assortment of
chromosomes
– Sexual reproduction creates genetic variation
– gametes of offspring do not have same combination of genes
as gametes from parents
• Note: Not know during Mendel’s time, random assortment in
humans produces 223 (8,388,608) different combinations in gametes
from Dadfrom Mom offspring
new gametes
made by offspring
3.3 U.7 Crossing over and random orientation promotes genetic
variation.

Fertilization Sources of Genetic Variation
– Human Sperm + Human Egg come together to produce
8 million X 8 million = 64 trillion combinations!
3.3 U.8 Fusion of gametes from different parents promotes genetic
variation.
http://news.bbcimg.co.uk/media/images/63005000/jpg/_63005
026_p6480029-sperm_fertilizing_egg-spl.jpg
http://worms.zoology.wisc.edu/dd2/echino/fert/files/page19_2.gif

• Sperm + Egg = ?
– any 2 parents will produce a zygote with over 70
trillion (223 x 223) possible diploid combinations
http://worms.zoology.wisc.edu/dd2/e
chino/fert/files/page19_2.gif
variation.

Sexual reproduction allows us to maintain both genetic
similarity & differences.
Baldwin brothers
Jonas
Brothers
http://upload.wikimedia.org/wikipedia/commons/f/ff/Jonas_Brothers_2009.jpg
Martin & Charlie Sheen, Emilio Estevez
variation.

3.3 A.1 Non-disjunction can cause Down syndrome and other
chromosome abnormalities.
Accidents During Meiosis Can Cause Chromosomal Abnormalities
– Nondisjunction: Chromosomes fail to separate.
– Gametes (and zygotes) will have an extra chromosome, others will be
missing a chromosome.
• Trisomy: Individuals with one extra chromosome, three instead of
pair. Have 47 chromosomes in cells.
• Monosomy: Missing a chromosome, one instead of pair. Have 45
chromosomes in cells.
http://upload.wikimedia.org/wikipedia/commons/3/31/Mitotic_nondisjunction.png

3.3 A.3 Description of methods used to obtain cells for karyotype
analysis e.g. chorionic villus sampling and amniocentesis and the
associated risks.
http://www.hopkinsmedicine.org/healthlibrary/GetImage.aspx?ImageId=161385
• Amniocentesis medical
procedure used in prenatal
diagnosis of chromosomal
abnormalities and also used
for sex determination.
• A small amount of amniotic
fluid, which contains fetal
tissues, is sampled from
the amniotic
sac surrounding a
developing fetus, and the
fetal DNA is examined for
genetic abnormalities.

3.3.A3 Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and
amniocentesis and the associated risks.
Can be carried out in the 16th week of the pregnancy with around
a 1% chance of a miscarriage
http://www.medindia.net/animation/amniocentesis.asp

3.3.A3 Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and
amniocentesis and the associated risks.
Can be carried out in the 11th week of the
pregnancy with around a 2% chance of a
miscarriage

3.3 A.2 Studies showing age of parents influences chances of non-
disjunction.
http://knowgenetics.org/wp-content/uploads/2012/12/DownSyndromeRisk.png
• Weakening of
cohesive ties holding
together
chromosomes in sex
cells may contribute
to maternal age-
related errors.
• The loss of cohesion
may contribute to
incorrect microtubule
attachment during
meiotic divisions

3.4 Inheritance
Essential Question: The inheritance of genes follows patterns.
http://upload.wikimedia.org/wikipedia/commons/1/11/Peas_in_pods_-_Studio.jpg

Understandings
Statement
3.4.U.1
Mendel discovered the principles of inheritance with experiments in which large numbers
of pea plants were crossed.
3.4 U.2 Gametes are haploid so contain only one allele of each gene.
3.4 U.3
The two alleles of each gene separate into different haploid daughter nuclei during
meiosis.
3.4 U.4
Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the
same allele or different alleles.
3.4 U.5
Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint
effects.
3.4 U.6
Many genetic diseases in humans are due to recessive alleles of autosomal genes,
although some genetic diseases are due to dominant or co-dominant alleles.
3.4 U.7
Some genetic diseases are sex-linked. The pattern of inheritance is different with sex-
linked genes due to their location on sex chromosomes. [Alleles carried on X
chromosomes should be shown as superscript letters on an upper case X, such as Xh.]
3.4 U.8 Many genetic diseases have been identified in humans but most are very rare.
3.4 U.9
Radiation and mutagenic chemicals increase the mutation rate and can cause genetic
diseases and cancer.

Statement Guidance
3.4 A.1
Inheritance of ABO blood groups. [The expected notation for ABO
blood group alleles: O = i, A=IA, B = IB.]
3.4 A.2
Red-green color blindness and hemophilia as examples of sex-
linked inheritance.
3.4 A.3 Inheritance of cystic fibrosis and Huntington’s disease.
3.4 A.4
Consequences of radiation after nuclear bombing of Hiroshima and
accident at Chernobyl.
3.4 S.1
Construction of Punnett grids for predicting the outcomes of
monohybrid genetic crosses.
3.4 S.2
Comparison of predicted and actual outcomes of genetic crosses
using real data. (Penny Lab)
3.4 S.3
Analysis of pedigree charts to deduce the pattern of inheritance of
genetic diseases.

Gregor Mendel
• Austrian monk who
published results of garden
pea plants inheritance in
1865
• Used artificial pollination in
a series of experiments by
using a small brush to place
the pollen on the
reproductive parts of the
flowers
3.4 U.1 Mendel discovered the principles of inheritance with experiments
in which large numbers of pea plants were crossed.

Key terminology
1. Genotype – symbolic representation of pair of alleles possessed by
an organism, typically represented by two letters
• Ex: Bb, GG, tt
2. Phenotype – characteristics or traits of an organism
• Ex: five fingers on each hand, color blindness, type O blood
3. Dominant allele – an allele that has the same effect on the
phenotype whether it is paired with the same allele or a different
one; always expressed in phenotype
• Ex: Aa give dominant trait A because the a allele is masked; the
a allele is not transcribed and translated during protein
synthesis
3.4 U.1 Mendel discovered the principles of inheritance with
experiments in which large numbers of pea plants were crossed.

4. Recessive allele – an allele that has an effect on the
phenotype only when present in the homozygous state
• Ex: aa gives rise to the recessive trait because no
dominant allele is there to mask it
5. Codominant allele – pairs of alleles that both affect the
phenotype when present in a heterozygote
• Ex: parent with curly hair and parent with straight
hair can have children with different degrees of
curliness as both alleles influence hair condition
when both are present in the genotype
6. Locus – particular position on homologous chromosomes
of a gene
3.4 U.1 Mendel discovered the principles of inheritance with experiments in
which large numbers of pea plants were crossed.

7. Homozygous – having two identical alleles of a gene
Ex: AA is a genotype which is homozygous dominant
whereas aa is the genotype which is homozygous
recessive
8. Heterozygous – having two different alleles of a gene
Ex: Aa is a heterozygous genotype
9. Carrier – an individual who has a recessive allele of a
gene that does not have an effect on their
phenotype

10. Test cross – testing a suspected heterozygote plant
or animal by crossing it with a known homozygous
recessive (aa). Since a recessive allele can be
masked, it is often impossible to tell if an organism
is AA or Aa until they produce offspring which
have the recessive trait.

3.4 A.1 Inheritance of ABO blood groups. [The expected notation for
ABO blood group alleles: O = i, A=IA, B = IB.]
There are 4: A, B, AB and O
A & B refer to 2 genetically
inherited A and B antigens on
the surface of red blood cells.
IA – codes for A
IB – codes for B
i - codes for no antigen = type
O blood

http://www.anatomybox.com/tag/erythrocytes/
3.4.U5 Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects.
Dominant alleles have the same effect on
the phenotype whether it is present in the
homozygous or heterozygous state
IAi
Type “O” allele present and blood
type is not O therefore the type “O”
allele is recessive to type “A”
Type “A” allele present and blood
type is A therefore the type “A”
allele is dominant to type “O”
IAIB Type “A” and “B” alleles are
present and blood type is AB
therefore type “A”and “B” alleles
are codominant
Codominant alleles are pairs of different
alleles that both affect the phenotype when
present in a heterozygote
Recessive alleles only have an effect on the
phenotype when present in the homozygous
state

Multiple Alleles: ABO Blood Groups
Blood type O: Universal donor. Blood type AB: Universal acceptor

Mendel’s Law of Segregation
Four parts
• Alternative versions of genes account for variations in inherited
characteristics.
• For each characteristic, an organism inherits two alleles, one from
each parent.
• If the two alleles differ, then one, the allele that encodes the
dominant trait, is fully expressed in the organism's appearance; the
other, the allele encoding the recessive trait, has no noticeable effect
on the organism's appearance.
• The two alleles for each characteristic segregate during gamete
production
3.4 S.1 Construction of Punnett grids for predicting the outcomes of

Principle of Segregation: Each Parent or Gamete
Contributes One Allele to Offspring

Punnet Square:
Used to determine the outcome of a cross between two individuals.
In the example we have two parents that are heterozygous
dominant for a trait
Offspring:
Genotype: 1/4 PP, 1/2 Pp, and 1/4 pp
Phenotype: 3/4 Purple and 1/4 white

3.4 S.2 Comparison of predicted and actual outcomes of genetic crosses
using real data.

F0
F1
Genotype: R ? r r
Phenotypes: All red
Test Cross Used to determine the genotype of an unknown individual.
The unknown is crossed with a known homozygous recessive.
Homozygous recessiveunknown
Phenotype:
Key to alleles:
R = Red flower
r = white
Some white, some red
Unknown parent = RR Unknown parent = Rr
Possible outcomes:
gametes r r
R Rr Rr
R Rr Rr
gametes r r
R Rr Rr
r rr rr
3.4.S2 Comparison of predicted and actual outcomes of genetic crosses using real data.

3.4 U.2 Gametes are haploid so contain only one allele of each gene.
• Gametes are sex cell.
• Sex cells contain one chromosome
of each type, as an example Humans
have 23 types.
• Parents pass information in
the form of genes in gametes (sex cell)
• These cell will fuse together with the
cell of the opposite sex to create a
zygote.

Meiosis = reduction
division
• Cells divide twice
• Result: 4 daughter
cells, each with half
as many
chromosomes as
parent cell
3.4 U.3 The two alleles of each gene separate into different haploid
daughter nuclei during meiosis.

Life cycle: reproductive history of organism, from
conception  production of own offspring
• Fertilization and meiosis alternate in sexual life cycles
• Meiosis: cell division that reduces # of chromosomes (2n 
n), creates gametes
• Fertilization: combine gametes (sperm + egg)
– Fertilized egg = zygote (2n)
• Zygote divides by mitosis to make multicellular diploid
organism
3.4 U.4 Fusion of gametes results in diploid zygotes with two alleles of
each gene that may be the same allele or different alleles.
.

Human Life Cycle
.

.

3.4 U.5 Dominant alleles mask the effects of recessive alleles but co-
dominant alleles have joint effects.
.
http://gestblog.scientopia.org/wp-content/uploads/sites/35/2012/07/10-04.gif

Problems with predictions
I. Codominance
II. Multiple alleles
III. Sex linked genes

http://image.slidesharecdn.com/incompletecodominancemultiplealleles-120127095123-phpapp02/95/incomplete-codominance-multiplealleles-7-728.jpg
3.4 U.6 Many genetic diseases in humans are due to recessive alleles of
autosomal genes, although some genetic diseases are due to dominant
or co-dominant alleles

Inheritance characterized by full expression of both alleles in
the heterozygote.
Seen in:
• Roan Cattle
• Tay Sacs disease
• Blood Types
You have a brown Bull and a
white Cow. You cross them and
get a mix of the two colors.
BB = Brown Bull/Cow
WW = white Bull/Cow
BW = Mixture of the two colors

https://jeanurquharthighlandsandislandsmsp.files.wordpress.com/2014/06/cavans-bourbon-orkney-by-robert_scarth-on-flickr.jpg

Sickle Cell Anemia: (example of a codomainant gene
mutation and its consequences through protein synthesis)
The Genetics of Sickle Cell Anemia
•HBA HBA Suceptible to malaria with anemia
• HBA HBs Increase resistance to malaria with mild anemia
• HBs HBs Sickle cell shaped cell Suceptible to malaria with
severe anemia

Sex Chromosomes
3.4 U.7 Some genetic diseases are sex-linked. The pattern of inheritance
is different with sex-linked genes due to their location on sex
chromosomes. [Alleles carried on X chromosomes should be shown as
superscript letters on an upper case X, such as Xh.]
• The X chromosome in humans
spans more than 153 million base
pairs (the building material
of DNA). It represents about 2000
out of 20,000 - 25,000 genes.
• The Y chromosome containing
78genes, out of the estimated
20,000 to 25,000 total genes in the
human genome. Genetic disorders
that are due to mutations in genes
on the X chromosome are
described as X linked.

Male sex chromosomes
• There are non-homologous
region males in which there
is only one allele per gene
and that is inherited from
the female on the X-
chromosome
• In the homologous region
the male inherited two
copies of an allele per gene.

Female sex chromosomes
• All regions of the X
chromosome are
homologous.
• There are two alleles per
gene as with all other genes
on all other chromosomes
This difference in x and y chromosomes plays a large role in
determining rates of genetic inherited defects

Sex Linkage Alleles on the non-homologous region of the
X chromosome are more common in females than in
males
• A gene with two alleles where one is dominant and one is recessive.
• Female has three possible genotypes and one is the homozygous
recessive.
• In a population the chance of being homozygous recessive is 33.3 %.
• Males have two possible genotypes.
• There is a 50% chance of the homozygous recessive condition in the
population.
• In sex linked conditions the recessive condition is more common in
males than females.

Female carriers of sex linked alleles
• Female heterozygote's for sex linked alleles e.g. Hemophilia
XHXh or Color Blindness XBXb are carriers of the allele.
• They are unaffected by the condition.
• They do pass on the allele which may result in a
homozygous female or a male with the sex linked recessive
allele.
3.4 A.2 Red-green color blindness and hemophilia as examples of sex-
linked inheritance.

Sex Linkage Examples:
Hemophilia
• Hemophilia is an example of a
sex linkage condition.
• The hemophilia allele is
recessive to the normal allele.
• The gene is located on the non-
homologous region of the X
chromosome.
• The disease is associated with
an inability to produce a clotting
factor in blood.
• Internal bleeding takes longer to
stop.
linked inheritance.
http://blog.nz-online.de/lieb/wp-content/uploads/sites/8/2010/07/blut.jpg

• The
homozygous
genotype(*) in
females has a
high mortality.
• The genotype
XnY in males
has a high
mortality.
linked inheritance.
Hemophilia
Hemophilia

• Red Green Color Blindness is an
example of a sex linked
condition.
• Red Green Color blindness is a
recessive condition.
• The color blind allele is recessive
to the normal allele.
• Female homozygous recessives
XbXb are color blind.
• Males with the genotype XbY are
color blind.
• Notice that in a population the
probability of having a Red Green
color blind genotype in males is
higher.
linked inheritance.
http://en.wikipedia.org/wiki/Color_blindness#/media/File:Ishihara_9.png
Above is a color test
plate.[The numeral "74"
should be clearly visible to
viewers with normal color
vision.

linked inheritance.
http://upload.wikimedia.org/wikipedia/commons/a/a3/XlinkRecessive.jpg
Normal color vision
Red/green color blindness

Pedigree Chart
• Another way to visualize a
monohybrid crosses or
determining a genotype is by
using a pedigree chart
• Knowing the phenotype of
individuals in a family will
sometimes allow genotypes to
be determined.
• In genetic counseling this
enables probabilities to be
determined for the inheritance
of characteristics in children.
3.4 S.3 Analysis of pedigree charts to deduce the pattern of inheritance
of genetic diseases.

Pedigree Chart
• White circle : Normal
female
• White Square: Normal
male
• Black Circle: affected
female
• Black square: affected male
• (1) and (2)..Normal Parents
• (3) affected female
• (4),(5) and (6) normal

1. Phenylketonuria (Pku)
• Using the allele key provided
state the genotype of parents
1 and 2?
• Give the genotype and
phenotype of individual 5 ?
• Is it possible that the
condition is sex linked ?
• What is the genotype and
phenotype of individuals 7
and 8?
• Which two individuals have
the incorrect pedigree

2. Muscular Dystrophy
• What type of genetic disease
is muscular dystrophy?
phenotype of 1?
phenotype of 2?
phenotype of 8 ?
phenotype of 5 and 6 ?

Cystic fibrosis (CF) Non Sex link recessive genetic trait found
on Chromosome 7
Example: Cross
The couple below are heterozygous for CF
http://www.bbc.co.uk/staticarchive/088e5fc50b3c51cfb49ebc4b6eaf203b18b93bbc.gif

Huntington’s Disease Non Sex link dominant genetic trait
The couple two couples below are examples
• Couple 1: 1 heterozygous (has trait) with 1 homozygous (without the
trait)
• Couple 2: Both parents are heterozygous with Huntington's
http://vanhornhuntingtonsdisease.weebly.com/uploads/1/3/7/4/13740905/4993818.jpg?1347964948
Couple 1 Couple 2

3.4 U.8 Many genetic diseases have been identified in humans but most
are very rare.
• Medical research has identified
over 4,000 genetic diseases,
however many individuals do
not suffer from one.
• Most genetic diseases are
caused by rare recessive
alleles. Making the chance of
inheritance very small.
• Genetic sequencing of the
human genome current
estimates are that there
maybe as little as 75-200
genes out of over 20,000
genes in the genome that
contain these traits.

Review: 1.6.U6 Mutagens, oncogenes and metastasis are involved in the development of primary
and secondary tumors.
mutation in a oncogene
If a mutation occurs in an oncogenes it can become cancerous. In normal cells
oncogenes control of the cell cycle and cell division.
http://en.wikipedia.org/wiki/Oncogene#mediaviewer/File:Oncogenes_illustration.jpg
uncontrolled cell division
tumor formation
malfunction in the control
of the cell cycle

3.4 U.9 Radiation and mutagenic chemicals increase the mutation rate
and can cause genetic diseases and cancer.
• A mutagen is a physical
(radiation), viruses, or
ultraviolet light. They can
also be chemical agent like
Nitrosamines, found in
tobacco. These mutagens
change the genetic
material, usually DNA, of
an organism and increases
the frequency
of mutations above the
natural background level.
Many mutations
cause cancer, mutagens are
therefore also likely to
be carcinogens.

• Radiation-induced cancers do not appear until at least 10 years after exposure (for
tumors) or 2 years after exposure (for leukemia).
• The risk of cancer after exposure can extend beyond this latent period for the rest
of a person’s life for tumors or about 30 years for leukemia.
• Risk is calculations are based on:
– The type of radiation.
• Each type of radiation is different and affects tissues differently.
– The energy that it leaves in the body.
• More energy means a higher probability of an effect.
– Where in the body the energy remains.
• Radiation exposure to a non-sensitive area of the body (i.e., wrist) really
has no actual effect. Radiation exposure to a sensitive area of the body (i.e.,
blood-forming organs) can have an effect if the amount of energy left is
high enough.

• Indirect damage
– Water molecule is ionized, breaks apart,
and forms OH free radical.
– OH free radical contains an unpaired
electron in the outer shell and is highly
reactive: Reacts with DNA.
– 75 percent of radiation-caused DNA
damage is due to OH free radical.
• Direct damage
– DNA molecule is struck by radiation,
ionized, resulting in damage.

Chromosome Damage
Formation of a ring and fragments followed
by replication of chromosomes.

Chromosome Damage
Interchange between two chromosomes
forms a chromosome with two centromeres
and fragment, followed by replication.

Commonly Encountered Radiation Doses
Effective Dose Radiation Source
<= 0.01 rem annual dose living at nuclear power plant panoramic, or full-mouth dental
x rays; skull or chest x ray
<=0.1 rem single spine x ray; abdominal or pelvic x ray; hip x ray; mammogram
<=0.5 rem kidney series of x rays; most barium-related x rays; head CT; any spine x-ray
series; annual natural background radiation dose; most nuclear
medicine brain, liver, kidney, bone, or lung scans
<=1.0 rem barium enema (x rays of the large intestine); chest, abdomen, or pelvic CT
<=5.0 rem cardiac catheterization (heart x rays); coronary angiogram (heart x rays);
other heart x-ray studies; most nuclear medicine heart scans
CT = computerized tomography; a specialized x-ray exam.
3.4 A.4 Consequences of radiation after nuclear bombing of Hiroshima
and accident at Chernobyl.

Radiation Doses and Expected Effects (cont.)
General radiation doses to the entire body and expected effects:
• 100-200 rem received in a short time will cause nausea and fatigue.
• 100-200 rem received over a long period will increase a person’s chances of getting cancer.
• 200-300 rem received in a short time will cause nausea and vomiting within 24-48 hours.
Medical attention should be sought.
• 300-500 rem received in a short time will cause nausea, vomiting, and diarrhea within hours.
Loss of hair and appetite occurs within a week. Medical attention must be sought for
survival; half of the people exposed to radiation at this high level will die if they receive no
medical attention.
• 500-1,200 rem in a short time will likely lead to death within a few days.
• Greater than 10,000 rem in a short time will lead to death within a few hours.

http://inapcache.boston.com/universal/site_graphics/blogs/bigpicture/hiroshima_08_05/h17_04.jpg

http://www.nucleardarkness.org/include/nucleardarkness/images/cityonfire/hiroshima_after_02_full.jpg

3.4.A4 Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl.
nuclear bombing of Hiroshima
https://upload.wikimedia.org/wikipedia/commons/e/e9/The_patient%27s_skin_is_burned_in_a_pattern_corresponding_to_the_dark_porti
ons_of_a_kimono_-_NARA_-_519686.jpg
https://upload.wikimedia.org/wikipedia/commons/f/f6/Hiroshima_girl.jpg
• Elevated rate of Leukemia (with the
greatest impact in children and young
adults)
• Elevated rates of other cancers
• No evidence of stillbirth or mutations in
the children of those exposed to radiation
http://i.telegraph.co.uk/multimedia/archive/02446/hiroshima-bomb_2446747b.jpg

http://www.zap-actu.fr/wp-content/uploads/2013/11/pripyat-une-des-villes-fantomes-pres-de-tchernobyl-01.jpg

3.4.A4 Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl.
accident at Chernobyl nuclear power station
https://upload.wikimedia.org/wikipedia/commons/1/16/VOA_Markosian_-_Chernobyl02.jpg
• A large area of pine forest downwind of
the reactor turned brown and died.
• Horses and cattle near the plant died
from radiation damage to their thyroid
glands.
• Bioaccumulation of radioactive caesium
in fish (Scandinavia and Germany) and
lamb (Wales) - contaminated meat was
banned from sale for years afterward.
http://i.guim.co.uk/img/static/sys-images/Guardian/Pix/pictures/2014/6/27/1403890449199/933cb303-bf75-4e9a-8b0b-806bbfa6a37b-
2060x1373.jpeg?w=620&q=85&auto=format&sharp=10&s=abe2802021d01fe090859454e9020a44
• Drinking water (and milk) contaminated with radioactive iodine - at least 6,000
thyroid cancer attributed to radioactive iodine.
• No clear evidence to support an increase in the rate of leukemia other cancers – in
part due to the widely dispersed variable radiation and measures taken in
European populations.

10.2 Inheritance
Essential idea: Genes may be linked or unlinked and are
inherited accordingly.
http://upload.wikimedia.org/wikipedia/c
ommons/4/46/Dividing_Cell_Fluorescenc
e.jpg
http://ies.rayuela.mostoles.educa
.madrid.org/deptos/dbiogeo/recu
rsos/Apuntes/ApuntesBioBach2/i
magenes/genetica/droso.png

Understandings
Statement Guidance
10.2 U.1 Gene loci are said to be linked if on the same chromosome.
10.2 U.2 Unlinked genes segregate independently as a result of meiosis.
10.2 U.3 Variation can be discrete or continuous.
10.2 U.4 The phenotypes of polygenic characteristics tend to show
continuous variation.
10.2 U.5 Chi-squared tests are used to determine whether the difference
between an observed and expected frequency distribution is
statistically significant.

Statement Guidance
10.2 A.1 Morgan’s discovery of non-Mendelian ratios
in Drosophila.
10.2 A.2 Completion and analysis of Punnett squares for
dihybrid traits.
10.2 A.3 Polygenic traits such as human height may also
be influenced by environmental factors.
10.2 S.1 Calculation of the predicted genotypic and
phenotypic ratio of offspring of dihybrid
crosses involving unlinked autosomal genes.
10.2 S.2 Identification of recombinants in crosses
involving two linked genes.
Alleles are usually shown side by
side in dihybrid crosses, for
example, TtBb. In representing
crosses involving linkage, it is
more common to show them as
vertical pairs,
10.2 S.3 Use of a chi-squared test on data from dihybrid
crosses.

10.2 U.1 Gene loci are said to be linked if on the same chromosome.
• Genes have specific locations (loci) on chromosomes
• Chromosomes segregate and assort independently
http://web.csulb.edu/~kmacd/361-6-Ch2.htm

10.2 U.2 Unlinked genes segregate independently as a result of meiosis.
• Mendel’s law of independent
assortment states allele pairs
separate independently from
other allele
pairs during gamete formation
(meiosis).
• Therefore, traits on different
chromosomes are transmitted
to the offspring independently
of traits on other
chromosomes.
• An exception to this rule is
linked genes
https://biologywarakwarak.files.wordpress.com/2011/12/crossingover-rec.jpg

10.2 U.3 Variation can be discrete or continuous.
• Discrete Variation is controlled by alleles of
a single gene or a small number of genes.
The environment has little effect. In this case
you either have the characteristic or you
don't. Cystic fibrosis is a good example for
this; either you have cystic fibrosis or you
don’t.
• Chi-squared calculations work well when
using examples with discrete variation
• Continuous Variation is a complete range of
phenotypes that can exist from one extreme
to the other. Height is an example of
continuous variation. Continuous variation is
the combined effect of many genes (known
as polygenic inheritance) and is often
significantly affected by environmental
influences. Skin color is another example of
continuous variation. http://articles.latimes.com/2013/nov/11/entertainment/la-et-
cm-princess-bride-disney-musical-20131111

Example: Human skin color
• This is controlled by as many as
6 genes each with its own
alleles.
• As the number of genes
increases the amount of
phenotypic variation increases.
• The alleles control the
production of melanin which is
a pigment that colors skin.
• In this example the calculation
is performed with 3 genes each
with 3 alleles. The cross is
between two individuals
heterozygous at both alleles
Allele Key
A= add melanin
a= no melanin added
B= adds melanin
b= no melanin added
http://www2.estrellamountain.edu/faculty/farabee/BIOBK/BioBookgeninteract.html

10.2 A.3 Polygenic traits such as human height may also be influenced
by environmental factors.
• Polygenic inheritance occurs when two or more genes control the expression of
a phenotype.
• As the amount of genes that control one trait increase, the number of phenotypes increases.
• Each additional gene has an additive affect, increasing the phenotypes. This is called
• Example: human height, which varies from person to person within the same race, and varies
between identical twin . Below the effect of environmental facts on the height of these
identical twin.
http://http://www.dailymail.co.uk/femail/article-2591087/Double-Poignant-portraits-50-
year-old-identical-twins-reveal-differences-lifestyle-affected-way-age.html

Mendel's Second Law of Independent Assortment
• This law states that allele pairs separate independently during the
formation of gametes.
• Therefore, traits are transmitted to offspring independently of one
another
10.2 A.2 Completion and analysis of Punnett squares for dihybrid traits.

Dihybrid Crosses Consider two traits, each carried on separate
chromsomes (the genes are unlinked).
Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
What is the predicted phenotype ratio for a cross between
two pea plants which are heterozygous at both loci?
F1
gametesPunnet Grid:
F0
Genotype:
Phenotype:
Heterozygous at both loci Heterozygous at both loci
10.2.A2 Completion and analysis of Punnett squares for dihybrid traits. AND 10.2.S1 Calculation of the predicted
genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametesPunnet Grid:
F0
Genotype: SsYy SsYy
Phenotype:
Smooth, yellow Smooth, yellow

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes SY Sy sY sy
SY SSYY SSYy SsYY SsYy
Sy SSYy SSyy SsYy Ssyy
sY SsYY SsYy ssYY ssYy
sy SsYy Ssyy ssYy ssyy
Punnet Grid:
F0
Genotype: SsYy SsYy
Phenotype:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes SY Sy sY sy
Punnet Grid:
F0
Genotype: SsYy SsYy
Phenotype:
Phenotypes: 9 Smooth, yellow : 3 Smooth, green : 3 Rough, yellow : 1 Rough, green

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
Calculate the predicted phenotype ratio for:
F1
Punnet Grid:
F0
Genotype:
Phenotype:
Heterozygous at both loci Heterozygous for S, homozygous dominant for Y
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
Punnet Grid:
F0
Genotype: SsYy SsYY
Phenotype:
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes SY sY
SY
Sy
sY
sy
Punnet Grid:
F0
Genotype: SsYy SsYY
Phenotype:
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes SY sY
SY SSYY SsYY
Sy SSYy SsYy
sY SsYY ssYY
sy SsYy ssYy
Punnet Grid:
F0
Genotype: SsYy SsYY
Phenotype:
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes SY sY
SY SSYY SsYY
Sy SSYy SsYy
sY SsYY ssYY
sy SsYy ssYy
Punnet Grid:
F0
Genotype: SsYy SsYY
Phenotype:
Phenotypes: 3 Smooth, yellow : 1 Rough, yellow Present the ratio in the simplest
mathematical form.
6 Smooth, yellow : 2 Rough, yellow

Dihybrid Crosses Common expected ratios of dihybrid crosses.
SY Sy sY sy
Heterozygous at
both loci
Heterozygous at
both loci
SsYySsYy
9 : 3 : 3 : 1
SY sY
SY SSYY SsYY
Sy SSYy SsYy
sY SsYY ssYY
sy SsYy ssYy
Heterozygous at
both loci
Heterozygous at one locus,
homozygous dominant at the other
SsYySsYy
3 : 2
Sy sy
SY SSYy SsYy
Sy SSyy Ssyy
sY SsYy ssYy
sy Ssyy ssyy
Heterozygous at
both loci
SsyySsYy
4 : 3 : 1
Heterozygous/
Homozygous recessive
ssYYSSyy = All SsYy
ssYySsyy = 1 : 1 : 1 : 1
ssyySSYY = all SyYy

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A rough yellow pea is test crossed to determine its genotype.
Punnet Grid:
F0
Genotype:
Phenotype: Rough, yellow
F1
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
Punnet Grid:
F0
Genotype: ssYy
F1
gametes sY sy
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes sY sy sY sYPunnet Grid:
F0
Genotype: ssYy or ssYY
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes sY sy sY sY
All sy
Punnet Grid:
F0
Genotype: ssYy or ssYYssyy
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes sY sy sY sY
All sy ssYy ssyy ssYy ssYy
Punnet Grid:
F0
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes sY sy sY sY
All sy ssYy ssyy ssYy ssYy
Punnet Grid:
F0
Phenotypes:
Some green peas will be present in
the offspring if the unknown parent
genotype is ssYy.
No green peas will be present in the
offspring if the unknown parent
genotype is ssYY.

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
A smooth green pea is test crossed. Deduce the genotype.
Smooth green = nine offspring. Rough green = one offspring.
F1
Punnet Grid:
F0
Genotype:
Phenotype: Smooth, green
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes
All sy
Punnet Grid:
F0
Genotype: ssyy
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes Sy Sy
All sy Ssyy Ssyy
Punnet Grid:
F0
Genotype: SSyyssyy
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes Sy Sy Sy sy
All sy Ssyy Ssyy Ssyy ssyy
Punnet Grid:
F0
Genotype: SSyy or Ssyyssyy
Phenotypes:

Key to alleles:
Y = yellow
y = green
S = smooth
s = rough
F1
gametes Sy Sy Sy sy
All sy Ssyy Ssyy Ssyy ssyy
Punnet Grid:
F0
Genotype: SSyy or Ssyyssyy
Phenotypes:
No rough peas will be present in the
offspring if the unknown parent
genotype is SSyy.
The presence of rough green peas in the
offspring means that the unknown genotype
must be Ssyy.
The expected ratio in this cross is 3 smooth green : 1 rough green. This is not the same as the outcome.
Remember that each reproduction event is chance and the sample size is very small. With a much larger
sample size, the outcome would be closer to the expected ratio, simply due to probability.

10.2 S.1 Calculation of the predicted genotypic and phenotypic ratio of
offspring of dihybrid crosses involving unlinked autosomal genes
Phenotype Key:
Phenotypic ratio:
– 9 Smooth Yellow:
– 3 Smooth green:
– 3 Rough Yellow:
– 1 Rough Green

10.2 S.1 Calculation of the predicted genotypic and phenotypic ratio of
offspring of dihybrid crosses involving unlinked autosomal genes

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